Neck muscles in the rhesus monkey. I. Muscle morphometry and histochemistry

Area of Science:

• Comparative anatomy
• Biomechanics
• Muscle physiology

Background:

• Understanding primate neck muscle architecture is crucial for biomechanical and evolutionary studies.
• Previous research has primarily focused on feline and human neck musculature, leaving gaps in primate-specific data.

Purpose of the Study:

• To morphometrically and histochemically characterize the neck muscles of adult Macaca mulatta (rhesus monkeys).
• To compare the architectural and fiber-type properties of rhesus monkey neck muscles with those of feline and human muscles.
• To elucidate the functional implications of these muscle specializations.

Main Methods:

• Morphometric analysis of musculotendinous lengths, fascicle lengths, pennation angles, and cross-sectional areas.
• Histochemical staining for ATPase activity to determine muscle fiber-type composition (Type I, IIa, IIb).
• Comparative analysis with existing data from feline and human neck muscles.

Main Results:

• Rhesus monkey neck muscles exhibit significant variability in architecture and fiber-type composition compared to feline and human muscles.
• Suboccipital muscles and those innervated by the spinal accessory nerve showed greater similarity to human homologs.
• Head-turning muscles had fewer fast-twitch (Type II) fibers than feline counterparts, while extensor muscles had more.
• Complex architectural features, including multiple attachments and internal aponeuroses, influenced cross-sectional area measurements.

Conclusions:

• Rhesus monkey neck muscles possess unique physical and histochemical specializations.
• These specializations directly influence functional properties like force production and fatigue resistance.
• Accurate biomechanical modeling and electrophysiological studies necessitate recognition of these specific muscle adaptations.